EXCHANGE 


8061' 
'A  'M 


AN  ELECTRIC  CONVERTER 


BY 

WM.   I.   BOOK 


A  THESIS 

PRESENTED  TO  THE  FACULTY  OF  PHILOSOPHY  Q,F  THE  UNIVERSITY  OF 

PENNSYLVANIA  IN  PARTIAL  FULFILLMENT  OF  THE  REQUIREMENTS 

FOR  THE  DEGREE  OF  DOCTOR  OF  PHILOSOPHY. 


PRESS  OF 

THE  NEW  ERA  PRINTING  COMPANY 
LANCASTER,  PA. 

1913 


AN  ELECTRIC  CONVERTER 


BY 

WM.  I.  BOOK 


.A  THESIS 

PRESENTED  TO  THE  FACULTY  OF  PHILOSOPHY  OF  THE  UNIVERSITY  OF 

PENNSYLVANIA  IN  PARTIAL  FULFILLMENT  OF  THE  REQUIREMENTS 

FOR  THE  DEGREE  OF  DOCTOR  OF  PHILOSOPHY 


PRESS  OF 

THE  NEW  ERA  PRINTING  COMPANY 
LANCASTER,  PA. 

1913 


[Reprinted  from  the  PHYSICAL  REVIEW,  N.S.,  Vol.  II.,  No.  i,  July,  1913.] 


AN   ELECTRIC   CONVERTER. 

BY  WM.  I.  BOOK. 

THIS  paper  describes  a  new  method  of  producing  electric  oscillations 
of  high  frequency  from  a  direct-current  source.     The  apparatus  is, 
therefore,  called  an  electric  converter. 

In  this  method  of  producing  electric  oscillations  the  primary  and 
secondary  oscillating  circuits  are  arranged  in  the  same  manner  as  they 
are  for  producing  electric  oscillations  with  the  Duddell  arc,  and  with  the 
Poulsen  arc.  But  the  converter  differs  much  from  all  other  methods 
previously  used  to  produce  continuous  electric  oscillations. 

DESCRIPTION  OF  APPARATUS. 

The  converter  consists  of  a  powerful  electro-magnet  with  heavy 
tubular  pole  pieces,  a,  about  7.6  cm.  in  diameter.  (See  Fig.  I.)  The 
walls  of  these  pole  pieces  are  reamed  out,  or  tapered,  at  the  end  to  a 
thickness  of  about  0.45  cm.  Fitted  on  the  end  of  each  of  these  pole 
pieces  is  a  fiber  ring,  &,  about  10  cm.  in  outer  diameter.  To  this  fiber 
insulation  is  attached  a  copper  (cathode)  ring,  r,  about  5.6  cm.  in  inner 
diameter  (to  be  described  below) .  One  of  the  tubular  pole  pieces  is  filled 
with  a  plug  of  fiber,  c.  A  brass  rod,  d,  neatly  fits  into  a  longitudinal 
hole  through  the  center  of  this  fiber  plug.  This  rod  is  thus  insulated 
from  the  pole  pieces.  There  is  fastened  to  the  end  of  this  rod  by  means 
of  a  machine  screw  a  zinc  or  carbon  (anode)  disc,  e,  about  5.4  cm.  in 
diameter.  The  faces  of  the  fiber  rings,  mentioned  above,  project  slightly 
beyond  the  ends  of  the  pole  pieces,  making  sockets  into  which  are  fitted 
discs  of  mica,  g,  over  the  ends  of  the  pole  pieces.  These  mica  discs 
with  the  fiber  rings  insulate  both  the  anode  disc  and  the  cathode  ring 
from  the  pole  pieces. 

If  a  difference  of  potential  of  approximately  500  volts  is  impressed 
across  the  gap  between  the  anode  disc  and  the  cathode  ring,  under  the 
proper  conditions,  a  spark  or  discharge  will  take  place  across  the  gap. 
Now  if  there  is  a  current  in  the  coil  of  the  electro-magnet  there  is  a 
powerful  magnetic  field  between  the  poles  of  the  magnet;  the  lines  of 
force  of  the  magnetic  field  passing  chiefly  through  the  gap  between  the 
anode  disc  and  the  cathode  ring.  The  direction  of  the  spark,  therefore, 
is  perpendicular  to  this  magnetic  field,  and,  in  accordance  with  the  funda- 


d  d  ,1  O  f)  f\ 


r SECOND 
rO  WM.  I.  BOOK.  [SERIES. 

mental  principle  which  underlies  the  action  of  an  electric  motor,  there  is 
a  force  action  on  it  perpendicular  to  both  the  magnetic  field  and  the 
direction  of  the  spark  discharge.  This  causes  the  discharge  to  move 
around  and  around  the  circular  gap  between  the  anode  disc  and  the 
cathode  ring.  But  either  the  anode  disc  or  the  cathode  ring  must  have 
a  rough  surface  in  order  to  obtain  the  conditions  necessary  to  produce 
oscillations  in.  an  oscillatory  circuit  shunted  about  the  spark-gap.  It 


was  found  best  to  put  a  regularly  roughened  surface  on  the  cathode  ring. 
Different  methods  were  tried  for  producing  this  regularly  roughened 
surface.  The  simplest  and  most  successful  method  tried  was  carefully 
to  wind  a  soft  copper  wire  of,  say,  No.  16  gauge  on  a  ring  made  of  No.  10 
or  No.  ii  gauge  wire.  This  wire-wound  ring,  r,  was  then  soldered  into 
a  circular  hole  cut  in  a  sheet  of  copper,  this  sheet  of  copper  serving  not 
only  as  a  conductor  to  which  the  line  from  the  negative  brush  of  the 
dynamo  is  attached,  but  also  as  a  heat  radiator  for  the  cathode  ring, 
thus  keeping  it  reasonably  cool. 

CONNECTIONS  OF  APPARATUS. 

In  Fig.  2,  there  is  shown  a  general  diagram  of  the  connections  of  the 
apparatus.  The  source  of  difference  of  potential  used,  (£),  was  two  J£ 
horse-power  500  volt  D.C.  dynamos  joined  in  parallel.  RQ  is  a  variable 
resistance  consisting  of  a  bank  of  carbon  filament  lamps  in  series  with  a 
slide-wire  contact  resistance  capable  of  varying  the  main  current  from 
about  i /i o  of  an  ampere  to  about  I  ampere.  70  is  a  direct  current  milli- 
ammeter  with  which  to  measure  the  current  in  the  main  circuit.  LQ  is  a 


>L.    II.1 
).  I.      J 


VOL.  II.l 
No. 


AN   ELECTRIC   CONVERTER. 


choke  coil.  The  high  tension  side  of  an  ordinary  electric  light  transformer 
was  used  for  the  choke-coil.  G  is  the  gap  between  the  anode  disc  and 
the  cathode  ring.  C\  is  a  variable  capacity  in  oil.  Cz  is  a  similar  capacity 
in  the  secondary  oscillating  circuit.  LI  is  the  primary  of  a  closely  wound 
induction  coil.  Lz  is  the  secondary  of  this  coil,  /i  is  a  hot  wire  ammeter 
with  which  to  measure  the  current  in  the  primary  oscillating  circuit. 
R2  is  a  variable  resistance  consisting  of  a  bank  of  carbon  filament  lamps. 


Fig.  2. 

THEORY  AND  PRINCIPLES  OF  THE  ACTION  OF  THE  SPARK-GAP. 

When  the  apparatus  is  connected  as  shown  in  the  diagram,  Fig.  2, 
and  when  the  proper  conditions  are  produced  for  the  spark  discharge  to 
take  place,  the  spark  will  pass  around  and  around  in  the  gap  between  the 
inside  disc  and  the  outside  regularly  roughened  or  notched  ring.  Because 
of  the  notches  or  grooves  in  the  outside  ring  the  sparking  distance  alter- 
nates rapidly  from  a  given  minimum  to  a  given  maximum  as  the  dis- 
charge passes  around  the  gap;  and,  therefore,  the  resistance  of  the  gap 
likewise  varies  with  this  varying  length.  The  function  of  the  choke  coil 
in  the  main  circuit  now  appears.  It  serves  to  maintain  a  nearly  constant 
supply  of  energy  through  the  gap  and  the  primary  oscillating  circuit 
shunted  around  the  gap.  When  the  spark  is  passing  between  the  inside 
disc  and  a  ridge  on  the  outside  ring  the  resistance  across  the  gap  has  a 
minimum  value,  but  when  the  spark  is  passing  between  the  disc  and  a 
groove  in  the  outside  ring  the  resistance  of  the  gap  is  a  maximum.  There 
will,  therefore,  be  a  rapdily  alternating  flow  and  ebb  of  energy  into  the 
primary  circuit, — at  each  high  resistance  in  the  gap  a  flow,  at  each  low 
resistance  in  the  gap  an  ebb.  Since  the  primary  oscillating  circuit  has  a 
natural  frequency  of  its  own,  it  would  seem  that  the  oscillations  in  this 
primary  circuit  for  a  given  set  of  conditions  will  have  an  increased 
amplitude, — and,  therefore,  a  greater  amount  of  energy  will  be  in  the 
primary  circuit, — when  the  natural  frequency  of  the  primary  oscillating 
circuit  is  an  integral  multiple  of  the  frequency  of  the  flow  and  ebb  of 
energy  into  this  circuit  due  to  the  alternating  higher  and  lower  resistances 
in  the  spark  gap. 


WM.  I.  BOOK. 


[SECOND 

[SERIES. 


A  number  of  variables  however  enter  into  the  investigation  of  these 
circuits  and  the  complete  determination  of  their  inter-relation  is  not  as 
simple  a  problem  as  at  first  it  seems  to  be.  The  D.C.  voltage  impressed 
across  the  gap,  the  amount  of  current  in  the  main  circuit,  the  average 
length  of  the  spark  gap,  the  number  of  ridges  and  grooves  in  a  given 
length  of  arc  of  the  spark  gap  circumference,  the  intensity  of  the  magnetic 
field  through  the  gap;  these  are  some  of  the  quantities  to  be  investigated. 

OBSERVATIONS  AND  DISCUSSION. 

The  spark-gap  may  vary  in  mean  length  with  different  conditions 
from  0.2  mm.  to  2.5  mm.  It  will  be  observed  at  once  that  with  a  spark 
gap  of  this  length  it  is  necessary  to  start  the  spark  discharge  by  some 
mechanical  means.  This  is  done  by  pushing  a  small  wire  into  a  groove 
in  the  fiber  next  to  the  cathode  ring  so  that  the  wire  touches  both  the 
inside  disc  and  the  outside  ring.  This  closes  the  main  circuit.  The 
wire  is  immediately  withdrawn,  thus  starting  the  spark  discharge. 

The  curves  of  Fig.  3  show  the  result  of  using  different  lengths  of  spark 
gap.  The  curve  marked  "Ztt»"  shows  the  variation  of  current  in  the 


*L  3 


800 


I 


190 


.*     *    .0    .0      J     JJt  24  J.G  J.8     2     33    7/l/ft. 

Length  of  Gap. 
Fig.  3. 

Variation  of  current  in  primary  with  change  of  spark-gap  length.  Variation  of  voltage 
across  gap  with  change  of  spark-gap  length.  Field  =  4,500  gauss,  /o  =  0.65  amp.,  C\ 
=  3.4X10-2  M.F. 

primary  oscillating  circuit  with  a  change  in  the  length  of  the  gap  when 
zinc  was  used  for  the  anode  disc.  The  curve  marked  "Ct-"  shows  the 
same  thing  for  carbon  as  the  anode  disc.  On  the  same  chart  are  shown 


VOL.  II.] 
No.  i.    J 


AN  ELECTRIC   CONVERTER. 


53 


two  curves  which  illustrate  the  variation  of  D.C.  voltage  across  the  gap 
with  a  change  of  spark  gap  length.  These  curves  are  marked  " Znv" 
and  "  Cv."  For  best  results  the  spark  gap  should  be  from  0.4  mm.  to 
1.5  mm.  in  length.  These  curves  seem  to  indicate  that  zinc  or  carbon 
may  be  used  for  the  anode  disc  with  approximately  the  same  results. 
Brass  and  aluminium  were  tried  with  practically  the  same  success. 

The  study  of  the  effect  of  a  varying  field  upon  the  current  in  the 
primary  oscillating  circuit  follows.  To  get  the  relation  between  these 
quantities  it  is  necessary  first  to  know  the  relation  between  the  current 
in  the  coil  to  produce  the  magnetic  field  across  the  gap,  and  the  field 
itself.  This  relation  was  determined  by  plotting  a  curve  with  the  current 
in  the  field  coil,  measured  in  amperes,  as  abscissas  and  with  the  field, 
measured  in  gausses,  as  ordinates.  From  this  curve  it  was  easy  to  note 
the  field  through  the  gap  corresponding  to  any  current  in  the  field  coil. 
The  method  used  to  determine  the  field  strength  was  the  method  of  the 
bismuth  spiral;  the  pole  pieces  being  kept  at  the  same  distance  from 
each  other  as  they  are  when  the  anode  and  cathode  parts  are  between 
them,  i.  e.,  at  a  distance  of  approximately  0.7  cm. 

If,  now,  the  spark  discharge  be  started  and  all  the  variable  quantities 
in  both  the  main  circuit  and  the  oscillating  circuits  be  kept  constant,  but 
the  field  be  increased  gradually,  the  current  in  the  primary  oscillating 
circuit  will  also  increase.  It  does  not,  however,  increase  as  a  straight 


JOO          /000        J.10O         ZOOO       MOO        3OOO       JSOO        WOO        4SOO       JOOO         Ge/(/SS 

Fig.  4. 

Current  curves  with  change  of  field  across  gap.    Ci  =  3.4  XIO"2  M.F.,  Vi  =  125  to  230  volts, 
depending  upon  length  of  gap.     Xi  =  330  m. 

line" function,  but  rises  to  a  maximum  above  which  an  increase  of  field 
will  not  further  augment  it.  If  the  field  is  increased  considerably 
beyond  the  value  which,  for  a  given  set  of  conditions,  will  give  the  maxi- 


54 


WM.  I.  BOOK. 


[SECOND 

[SERIES. 


mum  current  in  the  primary  oscillating  circuit  the  spark  discharge 
sputters  or  becomes  noisy  and  the  energy  in  the  oscillating  circuit  is 
decreased  rather  than  increased.  In  Fig.  4  are  shown  three  characteristic 
curves  illustrating  the  above  mentioned  result.  The  readings  for  the 
curve  marked  "Zn  2"  were  obtained  when  a  spark  gap  of  0.85  mm.  was 
used.  The  spark  gap  for  the  curve  "ZN  3"  was  1.3  mm.  long,  and  that 
for  the  curve  marked  " Zn  5"  was  2.3  mm.  in  length.  The  explanations 
of  this  result  that  have  suggested  themselves  to  the  writer  are  not  entirely 
satisfactory  to  him  and  are  reserved  for  further  verification. 

The  curve  of  Fig.  5  marked  II  shows  the  increase  of  current  in  the 
primary  oscillating  circuit  with  a  gradual  increase  of  the  capacity  in  that 
circuit.  The  curve  marked  70  shows  the  amount  of  current  in  the  supply 
circuit  corresponding  to  these  different  amounts  of  capacity  and  current 
in  the  primary  oscillating  circuit.  The  readings  for  the  curves  were  taken 
with  the  magnetic  field  across  the  gap  maintained  at  about  4,500  gausses. 


Capacity 

Fig.  5. 
Supply  circuit  and  oscillatory  circuit  currents  with  change  of  primary  capacity.      , 

To  get  the  maximum  current  in  the  primary  oscillating  circuit  for  a 
given  amount  of  capacity  in  this  circuit  the  current  in  the  main,  or  supply 
circuit  must  not  be  too  large.  When  there  is  too  much  current  in  the 
main  circuit  the  spark  discharge  sputters  and  is  irregular,  and  thus  little 
energy  gets  into  the  oscillating  circuits.  But  when  this  condition  is 
present,  if  the  supply  current  is  gradually  decreased  the  gap  runs  more 
and  more  smoothly,  and  the  current  in  the  primary  oscillating  circuit 
gradually  increases.  If  the  supply  current  is  still  gradually  decreased 
by  means  of  the  variable  resistance  in  the  main  circuit  the  current  in  the 
oscillating  circuit  continues  to  increase  until  it  comes  always  to  a  maxi- 


']  AN   ELECTRIC   CONVERTER.  55 

mum  value  for  the  given  capacity  in  the  circuit.  If  the  supply  current 
is  decreased  yet  a  trifle  the  spark  suddenly  stops — "blows  out."  This  is 
a  very  characteristic  phenomenon  of  the  converter.  It  seems  to  indicate 
that  as  the  energy  in  the  oscillating  circuit  becomes  greater  and  greater 
a  condition  is  reached  where  the  back  E.M.F.  impressed  across  the  gap 
due  to  the  oscillations  in  this  circuit  is  sufficient  to  reduce  the  D.C.  voltage 
across  the  gap  to  too  low  a  value  to  maintain  the  spark  discharge. 

The  number  of  notches  per  unit  length  of  arc  of  the  spark  gap  cir- 
cumference is  an  important  factor  in  determining  the  amount  of  energy 
that  is  transformed  in  the  oscillating  circuits.  A  very  large  number,  say 
25  per  centimeter  of  arc,  is  of  no  advantage,  but  rather  of  a  disadvantage. 
If  the  number  be  too  large  the  spark  discharge,  or  arc,  spreads  over 
several  of  them,  and  thus  a  regular  and  considerable  rise  and  fall  of  the 
spark  gap  resistance  is  not  obtained.  On  the  other  hand  if  the  number  of 
notches  be  too  few  then  the  number  of  pulses  per  unit  of  time  is  not  suffi- 
cient, with  a  low  field  across  the  gap,  to  maintain  the  oscillations  in  the 
primary  circuit.  However,  with  only  54  notches  in  the  entire  circum- 
ference of  the  gap  the  current  in  the  primary  increases  very  rapidly  as  the 
magnetic  field  across  the  gap  is  increased.  The  best  results  were  ob- 
tained with  a  cathode  ring  made  with  about  10  notches  per  centimeter  of 
arc. 

Among  the  observations  made  in  connection  with  the  secondary 
circuit  one  notable  fact  was  observed.  It  was  frequently  noticed  that 
when  the  gap  was  in  operation  with  slightly  too  much  current  in  the 
supply  circuit  and  when  the  secondary  was  not  tuned  to  the  primary,  the 
tuning  of  the  secondary  to  the  primary  produced  the  condition  in  the  gap 
which  would  be  obtained  by  gradually  decreasing  the  current  in  the 
supply  circuit;  i.  e.,  the  spark  discharge  becomes  more  regular  and  quiet 
and  the  current  in  the  primary  increases.  If  the  gap  is  already  acting 
smoothly  before  the  secondary  is  tuned  to  the  primary  an  attempt  to 
tune  it  will  cause  the  spark  to  "blow  out." 

APPLICATION. 

A  very  interesting  application  of  this  new  converter  occurs  in  wave- 
telegraphy.  The  writer  has  at  different  times  successfully  signalled  by 
means  of  it  to  a  private  wireless  station  three  city  blocks  distant.  These 
signals  were  very  distinctly  heard  at  stations  at  a  distance  of  from  three 
to  five  miles  from  the  Randal  Morgan  Laboratory  of  Physics  where  this 
research  was  pursued. 

There  are  several  ways  to  produce  the  signals  with  this  apparatus. 
One  method  is  to  join  in  series  with  the  secondary  circuit  (which  is  the 


56  WM.  L  BOOK. 

antenna  circuit  here)  a  suitable  resistance  of,  say,  carbon  filament  lamps 
and  shunt  this  resistance  with  the  signaling  key.  Upon  pressing  the  key 
to  produce  a  signal  the  resistance  is  shunted  and  the  energy  in  the 
secondary  is  radiated  through  the  antenna  instead  of  being  used  to  heat 
the  resistance.  Another  method  is  to  join  in  parallel  with  the  antenna 
circuit  a  secondary  which  can  be  tuned  to  the  primary.  These  two 
circuits  have  placed  in  them  a  double-action  key.  When  this  secondary 
circuit  is  closed  it  diverts  the  energy  from  the  antenna  circuit.  When 
the  key  is  pressed  to  produce  a  signal  the  tuned  secondary  circuit  is 
broken.  The  energy  then  "flows"  into  the  antenna  circuit  and  is 
radiated  by  it. 

It  is  the  confident  belief  of  the  writer  that  this  method  of  producing 
continuous  high  frequency  oscillations  will  succeed  not  only  in  wireless 
telegraphy  but  also  in  wave- telephony. 

SUMMARY. 

1.  A  result  of  this  research  is  that  an  electric  converter  to  produce 
continuous  electric  oscillations  of  high  frequency  has  been  constructed 
which  applies  the  fundamental  principle  of  the  electric  motor.     A  gas 
serves  as  the  conductor.     A  radial  spark  discharge  takes  place  through 
air  at  right  angles  to  an  intense  magnetic  field;  and  in  accordance  with 
the  electro-dynamical  principle  involved  there  is  a  force  action  on  the 
spark  perpendicular  both  to  the  direction  of  the  magnetic  field  and  to  the 
direction  of  the  spark  discharge,  causing  the  spark  to  move  rapidly 
around  in  a  circular  gap. 

2.  An  advantage  of  this  converter  consists  in  the  fact  that  the  spark 
discharge  takes  place  in  air  instead  of  in  some  other  gas  such  as  hydrogen. 

3.  To  produce  the  electric  oscillations  in  the  oscillatory  circuit  the 
circular  spark  gap  should  be  regularly  notched  with  about  10  notches 
per  centimeter  of  arc. 

4.  The  length  of  the  spark  gap  may  be  from  0.4  mm.  to  2.5  mm. 

5.  An  increase  of  the  magnetic  field  across  the  gap  increases  the  energy 
in  the  oscillatory  circuit  but  not  in  constant  ratio  with  the  increasing  field. 

6.  By  increasing  the  capacity  in  the  primary  oscillatory  circuit  the 
oscillating  current  is  rapidly  augmented,  while  the  increase  in  the  supply 
current  to  make  possible  such  increase  of  capacity  is  small. 

7.  Too  large  a  supply  current  decreases  the  energy  of  the  oscillatory 
circuit.     For  a  given  set  of  conditions  if  the  supply  current  be  gradually 
decreased  the  oscillating  current  increases  until  a  condition  is  reached 
where  further  decrease  of  supply  current  causes  the  spark  to  suddenly 
stop. 


V<J"X"']  AN  ELECTRIC   CONVERTER.  57 


No. 


8.  This  type  of  electric  converter  is  well  adapted  to  use  in  wave- 
telegraphy. 

In  conclusion  I  wish  to  thank  Professor  A.  W.  Goodspeed,  director  of 
the  Randal  Morgan  Laboratory  of  Physics,  for  placing  at  my  disposal 
the  facilities  of  the  laboratory  which  made  possible  this  investigation. 
I  also  wish  to  acknowledge  my  indebtedness  to  Dr.  R.  H.  Hough  for 
much  valuable  assistance,  and  for  suggesting  to  me  the  fundamental  idea 
from  which  this  research  developed. 

THE  RANDALL  MORGAN  LABORATORY  OF  PHYSICS, 
UNIVERSITY  OF  PENNSYLVANIA. 


UNIVEESITY  OF  CALIFORNIA  LIBRARY, 
BERKELEY 


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50c  per  volume  after  the  third  day  overdue,  increasing 
to  $1.00  per  volume  after  the  sixth  day.  Books  not  in 
demand  may  be  renewed  if  application  is  made  befoi'e 
expiration  of  loan  period. 


MAR  26  112? 


50m-8,'26 


UNIVERSITY  OF  CALIFORNIA  LIBRARY 


